A class D amplifier, also known as a switching amplifier, is an electronic amplifier that uses a switching mode of a switch, such as a transistor, to regulate power delivery. Class D amplifiers are preferable for many applications due to their high power efficiency.
The switching amplifiers usually contain a push-pull output stage with large sized transistors. One typical problem is the cross current, also known as shoot-through current, through the upper and bottom side of the amplifier. Usually each side contains only either metal oxide semiconductor (MOS) transistors of type n or p. The cross current is due to the fact that the switching of the transistors requires some time and during the transition there is a moment when the transistors of both sides are conducting simultaneously thereby introducing a short connection between a power supply and ground. This short connection becomes more critical for large sized output stages or high voltage supplies and may introduce a considerable current spike increasing the power consumption and heating of the devices. In some severe cases the cross current may even reduce the reliability and lifetime of a class D amplifier.
To avoid the cross current, a dead time between the switching has to be introduced. Setting a proper value for the dead time is always a compromise due to the dependence of the switching time on different factors such as temperature, technology variations, supply voltage, etc. However, increasing the dead time leads to a rise of undesired total harmonic distortions (THD). One typical output stage of a complementary metal oxide semiconductor (CMOS) technology implementation is presented in FIG. 1. FIG. 2 illustrates the definition of dead time in the configuration of the CMOS output stage of FIG. 1. With the rising edge of the input pulse width modulated (PWM) signal, the MN transistors are turned on, whereas with the falling edge of the PWM signal the MP transistors are switched on.
A common practice is to use a fixed dead time with some margins. However, this has the drawback that additional harmonic distortions are introduced. A typical dead time value is about 50-100 ns. Variable dead time can also be used. Current methods are mostly based on the sensing of the gate voltage of the output transistors of the amplifier (i.e. in case of FIGS. 1 and 2 transistors MN1, MN2, MN3, MP1, MP2 and MP3) and disabling the opposite side pre-driver (e.g. for the MN transistors the opposite side pre-driver would be the P_driver) until the active side is not completely or almost switched off. For the configuration of FIG. 3, this is illustrated in FIG. 4.
The comparator Comp_N will apply a blanking potential to N_Driver until the output potential of P_Driver reaches the reference voltage VTH1. The blanking potential will keep the output of N_Driver low, i.e. the MN transistors are switched off. The comparator Comp_P will apply a blanking potential (i.e. the MP transistors are switched off) to P_Driver until the output potential of N_Driver falls down below the reference voltage VTH2. The purpose of the blanking potential is thus to prevent the transistors from switching on.
After the end of the blanking potentials, the pre-drivers will be enabled and their outputs will start to execute the transition. One additional delay will be introduced due to the limited speed of the comparators. The limited speed of the comparators may bring a significant inaccuracy for some of the CMOS technologies, especially high voltage (HV) CMOS technologies often used in integrated power amplifiers. Another source of inaccuracy may be the matching of the blanking signal to the rising/falling time of the pre-driver, but not directly to the cross current spike in the output stage.
It is thus the object of the present invention to overcome the above-identified difficulties and disadvantages by proposing an improved solution for avoiding cross current in class D amplifiers.